Analysis of INS Derived Doppler Effects on Carrier Tracking Loop

Abstract

Tracking dynamics on the GPS signal is still a big challenge to the receiver designer as the operating conditions are becoming more volatile. Optimizing the stand-alone system for dynamics generally degrades the accuracy of measurements. Therefore, an inertial navigation system (INS) is integrated with GPS to address this issue. Doppler derived from INS can be used to aid the carrier tracking loop for improving the performance under dynamic conditions. However, the derived doppler does not truly reflect the GPS signal doppler due to errors in inertial sensors. As the tracking loop bandwidth is reduced significantly in ultratightly integrated systems, any offsets in the aiding doppler creates undesired correlations in the tracking loop resulting in sub-optimal performance of the loop. The paper addresses this issue and also provides a mitigating mechanism to reduce the effects of incorrect estimates of the doppler. It is shown that doppler offsets resulting in a bias in the tracking loop can be appropriately modelled and removed. Mathematical algorithms pertaining to this are provided and the results are summarized. Simulations show that the bias due to aiding doppler offsets could be effectively addressed by appropriate modelling. K E Y W O R D S 1. INS derived Doppler. 2. Correlations. 3. Stochastic Modelling. I N T R O D U C T I O N.Continuous tracking of GPS signals in dynamic scenarios pose a significant challenge for the design of the tracking loops. Optimizing a design to suit a particular scenario will degrade its performance in other scenarios. For instance, increasing the carrier tracking loop bandwidth to receive dynamic signals will inadvertently affect the accuracy of the raw measurements (Jwo, 2001 ;Cox, 1982). Therefore, in a stand-alone GPS receiver, a trade-off design is required to perform optimally in all the scenarios. External sensor integration with the GPS is considered as an alternative to improve upon this, and INS is the ideal choice as it is not only autonomous but also provides attitude at higher data rates. Traditionally, the integration of GPS and INS were carried out in loosely and tightly coupled configurations (Brown & Hwang, 1997). While these systems offer significant advantages over the stand-alone GPS, nevertheless it is imperative to improve the performance wherever possible. With this point of view, the development of integration presently culminated in ultra-tight systems. This type of integration, also called deep level tracking, integrates the I (in-phase) and Q (quadrature) signals

High Dynamic Carrier Tracking Using Kalman Filter Aided Phase-Lock Loop

“…The RMS error of the Doppler estimation is 1.31Hz. It has been show in [6] that, the EKF has a threshold about 24dB-Hz, and at CNR 26 dB-Hz, the RMS error of the Doppler estimation is 2.2Hz, which is smaller than the 5Hz error of the IMU used in [7]. So we can say that, this accuracy can be used to aid the tracking loop.…”

“…In [7], Doppler derived from IMU is used to aid the carrier tracking loop for improving the performance under dynamic conditions. Similarly, we use the EKF instead of the IMU to extract the Doppler signal to aid the PLL (Fig.…”

Improving MEMS-IMU/GPS integrated systems for land vehicle navigation applications

“…It is well known that the tightly coupled INS/GPS integration offers some unique capabilities, e.g., employment of GPS data even under absence of minimum requirements (i.e., direct visibility of four satellites, at least) or aiding carrier tracking loops (Babu and Wang 2005). Nonetheless, the most popular method for MEMS-INS/GPS integration is the loosely coupled algorithm, which lets the GPS work on its own algorithm and thus remains immune from noisy MEMS-IMU observations, in addition to reduction in system complexity.…”

Novel optimal bandwidth design in INS-assisted GNSS Phase Lock Loop

“…In INS-assisted receiver, velocity aiding can help to continue tracking GNSS pseudorandom code signal or carrier signal, and then receiver bandwidth can be adjusted to accommodate pre-selected INS [3,4]. This availability of accurate carrier phase measurements is deemed necessary for many high-accuracy applications, such as mobile mapping in urban areas, and indoor surveys with GNSS [5].…”